Download The Challenge of Distances (PowerPoint version)

The Distances of the
Spiral Nebulae:
A Huge Challenge!
The ‘Great Debate’
Curtis vs Shapley (1920):
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are the nebulae big, even comparable to our
own Milky Way (as Curtis argued)?
or are they small satellites of the Milky Way (as
Shapley maintained)?
Shapley’s arguments, on varied lines of evidence,
carried the day (but he was wrong).
How Can We Determine Their Distances?
A Naïve Approach
Why not just take some photographs that demonstrate that
the spiral nebulae are full of stars?
The extreme faintness of those stars would tell us that
even the largest, closest nebula (like the Andromeda
nebula, M31) is very far away. Surely that would resolve
the question!
Unfortunately it was not that simple.
Meet M31, Our Nearest Big Neighbour
Just visible (as a fuzzy patch)
to the unaided eye – the
most remote object we
can see directly.
The Problem
Andromeda (a.k.a. M31), the closest big galaxy, is
2 million light years away.
This makes the constituent stars very faint
At the very start of the 20th century, telescopes
and photographic techniques were simply not
able to resolve even the brightest giant stars in
M31
The dots of light you see sprinkled around the image are
foreground stars, in our own Milky Way and quite close
to us. Close examination of the M31 nebula itself does
not show myriads of member stars, just a general ‘blur’
of light.
How About Now?
Even now, the Hubble
Space Telescope
would just barely be
able to detect a sunlike star in M31, our
very nearest galaxy
neighbour.
Studying the stars in
more remote galaxies
is of course even
more challenging.
It’s Not Quite as Bad as it Sounds!
There are, of course, lots of stars that are much
brighter than the sun!
With big enough telescopes, these intrinsically brighter
stars (giants and supergiants) can be seen and
studied.
By the 1910’s, the 100-inch Hooker telescope – the
world’s largest – at Mt Wilson (Los Angeles) was
able to just detect the very brightest stars in M31.
The Hooker Telescope
A Second Problem, However
How do we convince ourselves that the dots of
light that we see in a nebula are stars, not just
(say) ‘Jupiters in formation’?
Here’s an Insufficient Answer
Suppose we study the spectrum of one of the dots of light, and
discover (say) that it is comparable to the spectrum of the Sun. This
will not prove that it is a star!
Why not? It might simply be reflecting the light from the hypothetical
‘protostar’ that is forming at the middle of the nebula.
(Analogy: here in the Solar System, the light we get from Jupiter has a
spectrum like the Sun: it’s reflected sunlight!)
Anyway, they are so faint that astronomers couldn’t get a spectrum,
which requires spreading out the light. So this approach was not
possible.
A Much Better Idea
Let’s monitor the nebula to see if one of the dots
of light behaves in a way that only a star does!
Exactly what do we mean by that? Should we
watch to see if the ‘star’ evolves? Of course not! –
that takes millions of years! We need a more
practical short-term approach.
The Eventual Solution:
Identify Variable Stars
It was discovered by Hubble that some of the
dots of light in M31 vary in brightness in a way
that only certain types of very bright stars do.
So they are clearly not ‘Jupiters in
formation,’and the nebulae are not solar
systems at birth.
As we will see, that settled the question: the
nebulae are galaxies like the Milky Way!